1. Field of the Invention
The invention relates to display devices. More particularly, the invention relates to liquid crystal display devices with improved display quality and luminance, and a method for fabricating the same.
2. Description of the Related Art
A liquid crystal display (hereinafter referred to as “LCD”) device is a type of flat panel display device. Like most flat panel display devices, LCD devices are smaller and lighter than cathode-ray tube (CRT) display devices of a comparable screen size. The smaller size and lower weight make flat panel displays advantageous in various applications.
LCD devices display information such as characters, images, and moving pictures, by using the optical properties of liquid crystal molecules. An electric field, which forms in the layer of liquid crystal molecules in response to voltage, changes the arrangement and/or the orientation of the liquid crystal molecules. This change in arrangement and/or orientation of the liquid crystal molecules, in turn, affects the transmissivity of light through the liquid crystal layer. Since the liquid crystal itself does not emit light, the LCD device requires a light source to display an image. Accordingly, the LCD device sometimes employs a separate light source.
The LCD device is classified into three categories: a transmissive type LCD device, a reflective type LCD device, and a transmissive-reflective type LCD device.
A transmissive type LCD device uses an internal light source for displaying an image. Thus, in transmissive type LCD devices, light that is generated by application of electrical power to a light source is provided to the liquid crystal layer for displaying information on a screen of the LCD device. In contrast, a reflective type LCD device uses light from a light source that is external to the LCD device. This external light source may be, for example, natural light such as sunlight or an artificial light such as light illuminated from a lamp. A transmissive-reflective type LCD device uses both an internal light source and an external light source for displaying an image. Typically, the transmissive-reflective type LCD device uses the internal light source when there is insufficient amount of external light and uses external light when there is a sufficient amount of it.
Both the reflective type and the transmissive-reflective type LCD devices include a reflective electrode to reflect the light from an external light source, thereby displaying images. Therefore, enlarging the reflective surface of the reflective electrode usually improves the front luminance of these LCD devices.
When the surface area of the reflective electrodes is maximized, the non-reflective area between the reflective electrodes decreases in the reflective type or the transmissive-reflective type LCD device. Recently, LCD devices have been made with reflective electrodes disposed on a signal line, wherein an electric power of a predetermined voltage is applied to the signal line. The reflective electrodes being disposed on the signal line is an advantage of the reflective type or the transmissive-reflective type LCD devices.
However, the reflective type or the transmissive-reflective type LCD device also suffers from a disadvantage that lowers the display quality. The reflective electrode disposed on the signal lines must be electrically isolated from the reflective signal lines. To achieve this electrical separation, an organic insulating layer having a dielectric constant of about 3.3 is interposed between the reflective electrode and the signal line. The surface of the insulating layer is generally patterned to direct light such that the front luminance is improved. Overall, the improvement in front luminance is due to an increase in the surface area through which the light passes, and the attendant increase in light diffusion. Since this improved luminance stems from the increased surface area of the reflective electrode, which in turn is made possible by the presence of the insulating layer, the insulating layer significantly affects the display quality of a reflective type or a transmissive-reflective type LCD device.
Generally, it is desirable to make the insulating layer thin because a thin insulating layer facilitates formation of contact holes and light transmission. However, when the insulating layer is too thin, a parasitic capacitor may form between the conductive signal line and the non-conductive insulating layer, and between the non-conductive insulating layer and the conductive reflective electrode. This formation of the parasitic capacitor is undesirable, as it distorts an electrical signal applied through the signal line and modifies the voltage applied to the reflective electrode. Ultimately, the parasitic capacitor deteriorates the display quality of the LCD device.
To reduce the parasitic capacitance, the thickness of the insulating layer can be increased. However, this thickening of the insulating layer adversely affects light transmission, as discussed above. Although parasitic capacitance can be reduced without thickening the insulating layer if the reflective electrode is not formed over the signal lines, this solution also has the undesired effect of reducing the reflective surface area of the reflective electrode, thereby still decreasing the luminance of the LCD device. Accordingly, every option has a disadvantage. Thus, there continues to be a need for novel structures of the LCD device and manufacturing methods that can improve the front luminance of an LCD device.